In this paper we deal with the problem of decentralized observability of discrete event systems. We consider a set of sites that observe a subset of events. Each site transmits its own observation to a coordinator that decides if the word observed belongs to a legal behavior or not. We study two different properties: uniform q-observability and q-diagnosability. Then, we prove that both properties are decidable for regular languages. Finally, we give an algorithm to compute starting from a given initial state, the time instants at which the synchronization has to be done so as to guarantee that if an illegal word has occurred it is immediately detected.